1. Field of the Invention
The present invention relates to a control device and a control method for a power supply device such as a DC/DC converter and an AC/DC converter, and more particularly to a control device and a control method for performing constant voltage constant current control.
2. Description of the Related Art
In the field of power supply such as a commercial power source, renewable energies like solar power generation and wind power generation are becoming widespread. Under the circumstances, a smart grid, a next-generation power infrastructure equipped with means for compensating mismatches between demand and supply through the use of secondary batteries, has been being built recently. In the field of automobiles, plug-in hybrid electric vehicles (PHEVs), electric vehicles (EVs), and the like of which secondary batteries can be charged with a commercial power source etc., as well as conventional hybrid vehicles (HVs) of which secondary batteries are charged by an engine, are growing rapidly in the market. The importance of the technique for storing electric power in a secondary battery is recognized once again, and research and development has been active in view of stable power supply and CO2 emission reduction.
Among such secondary batteries, lithium ion batteries are becoming mainstream instead of conventional lead batteries, nickel-cadmium batteries, nickel metal hydride batteries, etc. A lithium ion battery is high in energy density, small in size, and light in weight, and has no memory effect at high voltage. The lithium ion battery is characteristically prone to degradation if kept in a full charge state, and thus not suited for trickle charging. The lithium ion battery can possibly cause fire if overcharged. CVCC control, or constant voltage constant current control, for strictly managing both voltage and current is thus needed for charging. Specifically, control for preventing an overcharge and the like is performed, for example, by maintaining the charge current at a generally constant level (constant current control) in a low voltage state in an initial stage of charging, and maintaining the charge voltage at a generally constant level (constant voltage control) in a high-voltage state after the amount of charge has increased.
In the CVCC control, both the output voltage and the output current from the power supply need to be substantially simultaneously detected for feedback control. Control systems are broadly classified into two types, a cascade system and a parallel control system. In the cascade system, a CC control unit (constant current control unit) for performing CC control and a CV control unit (constant voltage control unit) for performing CV control are arranged in series. In the parallel control system, a CC control unit and a CV control unit are arranged in parallel. There are two types of cascade systems. One is a CV control minor loop/CV control major loop in which control is performed in order of CC control→CV control. The other is a CC control minor loop/CV control major loop in which control is performed in order of CV control→CC control.
The CV control minor loop/CV control major loop cascade system is mainly used to add a constant current control function to a commercially available constant voltage power supply device which is configured to be able to change its output voltage according to an external signal (for example, see Japanese Patent Application Laid-Open No. Hei 7-219652). A typical commercially available power supply device has an auxiliary function of changing its output voltage, called TRM function, PV function, VR function, etc., and is equipped with an input terminal of an external voltage adjustment signal. This terminal can be utilized to add a constant current function easily. Such a method is also described as an application in a catalog and the like.
FIG. 5 is a block diagram showing an example of a configuration of a CV control minor loop/CC control major loop cascade system.
The following description deals with an example where a single-transistor forward converter (DC/DC converter) having a known configuration is used as a power supply device 100. Specifically, the power supply device 100 includes a pair of input terminals 101, a pair of output terminals 102, a transformer 103, a switch 104, a driver 105, a rectifier circuit 106, a smoothing circuit 107, and an input capacitor 108. A direct-current voltage having an input voltage value of Vin is applied to the pair of input terminals 101. A load 200, such as a secondary battery, is connected to the pair of output terminals 102. The driver 105 switches on/off the switch 104 on the basis of a driving pulse P transmitted from a control device 110, whereby an output voltage value Vo and an output current value Io output to the load 200 are controlled.
The control device 110 of the CV control minor loop/CC control major loop cascade system includes a CC control unit 111, a CV control unit 112, and a driving pulse generation unit 113.
The CC control unit 111 compares a current target value Iref set by a target value setting unit 114 with the output current value Io detected by an output current detection unit 115. The CC control unit 111 then generates a voltage target value Vref by PID control or the like so that a current difference value ΔEi, which is a difference between the current target value Iref and the output current value Io, approaches zero. The voltage target value Vref is limited by a limiter 116 so as not to exceed a voltage target upper limit value V0ref set by the target value setting unit 114.
The CV control unit 112 compares the voltage target value Vref generated by the CC control unit 111 with the output voltage value Vo detected by an output voltage detection unit 117. The CV control unit 112 then generates an operation amount U by PID control or the like so that a voltage difference value ΔEv, which is a difference between the voltage target value Vref and the output voltage value Vo, approaches zero. The driving pulse generation unit 113 generates a driving pulse P by a PWM method for modulating a pulse width or the like on the basis of the operation amount U, and transmits the driving pulse P to the driver 105.
According to such a system, for example, in a low voltage state in an initial stage of charging where the output voltage value Vo is low, the voltage target value Vref is generated as a value for bringing the current difference value ΔEi close to zero. The control device 110 thus performs CC control. As the amount of charge increases, the output voltage value Vo increases. After the voltage target value Vref needed to bring the current different value ΔEi close to zero exceeds the voltage target upper limit value V0ref, the voltage target value Vref is maintained at the same value as the voltage target upper limit value V0ref by the limiter 116. The control device 110 consequently performs CV control.
A CC control minor loop/CV control major loop cascade system is implemented by interchanging the CC control unit 111 and the CV control unit 112 of the foregoing CV control minor loop/CC control major loop cascade system. The CC control minor loop/CV control major loop cascade system is applied to a bidirectional power supply device and the like for charging and discharging a secondary battery (for example, see Japanese Patent No. 5185328).
FIG. 6 is a block diagram showing an example of a configuration of the CC control minor loop/CV control major loop cascade system.
In such a system, the CV control unit 112 compares the voltage target value Vref set by the target value setting unit 114 with the output voltage value Vo detected by the output voltage detection unit 117. The CV control unit 112 generates the current target value Iref by PID control or the like so that the voltage difference value ΔEv, a difference between the voltage target value Vref and the output voltage value Vo, approaches zero. The current target value Iref is limited by the limiter 116 so as not to exceed a current target upper limit value I0ref set by the target value setting unit 114.
The CC control unit 111 compares the current target value Iref generated by the CV control unit 112 with the output current value Io detected by the output current detection unit 115. The CC control unit 111 generates the operation amount U by PID control or the like so that the current difference value ΔEi, a difference between the current target value Iref and the output current value Io, approaches zero. The driving pulse generation unit 113 generates the driving pulse P by a PWM method for modulating a pulse width or the like on the basis of the operation amount U, and transmits the driving pulse P to the driver 105.
According to such a method, in a state where the output current value Io is small, the current target value Iref is generated as a value for bringing the voltage difference value ΔEv close to zero. The control device 110 thus performs CV control. After the output current value Io increases and the current target value Iref needed to bring the voltage difference value ΔEv close to zero exceeds the current target upper limit value I0ref, the current target value Iref is maintained at the same value as the current target upper limit value I0ref by the limiter 116. The control device 110 consequently performs CC control.
In the parallel control system, the CC control unit 111 and the CV control unit 112 are arranged in parallel, and either one of CC control and CV control is performed selectively (for example, see Japanese Patent No. 4162546). FIG. 7 is a block diagram showing an example of a configuration of the parallel control system. In such a system, the control device 110 includes a target value selection unit 118 in addition to the CC control unit 111, the CV control unit 112, and the driving pulse generation unit 113. The CC control unit 111 compares the current target value Iref set by the target value setting unit 114 with the output current value Io detected by the output current detection unit 115. The CC control unit 111 generates a CC control operation amount Ui by PID control or the like so that the current difference value ΔEi, a difference between the current target value Iref and the output current value Io, approaches zero. Similarly, the CV control unit 112 compares the voltage target value Vref set by the target value setting unit 114 with the output voltage value Vo detected by the output voltage detection unit 117. The CV control unit 112 generates a CV control operation amount Uv by PID control or the like so that the voltage difference value ΔEv, a difference between the voltage target value Vref and the output voltage value Vo, approaches zero.
The target value selection unit 118 selects either one of the CC control operation amount Ui and the CV control operation amount Uv as the operation amount U. The driving pulse generation unit 113 generates the driving pulse P by a PWM method for modulating a pulse width or the like on the basis of the selected operation amount U, and transmits the driving pulse P to the driver 105. If the target value selection unit 118 selects the CC control operation amount Ui, the control device 110 performs CC control. If the target value selection unit 118 selects the CV control operation amount Uv, the control device 110 performs CV control.
In the conventional cascade systems, the target value of the minor loop is changed by the major loop. Suppose that the minor loop and the major loop have similar response speeds. In such a case, if the major loop changes the target value while the control of the minor loop is in operation, the control of the minor loop can become unstable. In the cascade systems, the response speed of the control of the major loop therefore needs to be sufficiently lower than that of the minor loop so that the target value will not vary during the control of the minor loop.
The responsiveness of the CC control is largely dependent on an impedance and the like of the load 200. If the CC control is the major loop, there can occur a problem due to the low response speed. To solve such a problem, the system needs to be tuned according to the state of the load 200. The result has been high costs and low versatility of the CV control minor loop/CC control major loop cascade system. If the CV control is the major loop, the response speed of the CV control, which is the most important to power supply devices, drops significantly. The uses of the CC control minor loop/CV control major loop cascade system have thus been limited to a bidirectional power supply device, a power factor improvement step-up converter, and the like of which high response speed is not needed.
In the parallel control system, the CC control unit 111 and the CV control unit 112 independently generate the CC control operation amount Ui and the CV control operation amount Uv, respectively. There occurs no problem like those of the cascade systems, and appropriate responsiveness can be provided both in the CC control and the CV control. However, according to the parallel control system, the CC control unit 111 and the CV control unit 112 constantly generate the CC control operation amount Ui and the CV control operation amount Uv regardless of which is selected as the operation amount U. There has thus been a problem of unstable control due to a change in the operation amount U when control is switched from CC control to CV control or from CV control to CC control.
Specifically, for example, during CC control, the output current value Io is close to the current target value Iref and the output voltage value Vo is deviated from the voltage target value Vref. The CV control operation amount Uv generated during the CC control is significantly greater than the CC control operation amount Ui mostly because of the influence of an integral operation. If the CC control is switched to CV control and the CV control operation amount Uv is selected as the control amount U, the output voltage and the output current vary greatly. There has thus been a problem that it takes long to converge to a stable state.
To address such a problem, the control method described in the foregoing Japanese Patent No. 4162546 includes cancelling the integral operation of the unselected control to provide stability at the time of switching of control. However, in the control method of the foregoing Japanese Patent No. 4162546, whether to cancel an integral operation needs to be determined on the basis of which of the CC control operation amount Ui and the CV control operation amount Uv is selected. There has thus been a problem of complicated control. Moreover, according to the control method of the foregoing Japanese Patent No. 4162546, the control circuit for generating the CC control operation amount Ui and the control circuit for generating the CV control operation amount Uv are separately provided. There has thus been a problem of increased installation space as well as an increase in the cost of the control device 110.